Inkjet printers, and thermal inkjet printers in particular, have come into widespread use in businesses and homes because of their low cost, high print quality, and color printing capability. These printers and related hardcopy devices are described by W. J. Lloyd and H. T. Taub in “Ink Jet Devices,” Chapter 13 of Output Hardcopy Devices (Ed. R. C. Durbeck and S. Sherr, San Diego: Academic Press, 1988). The operation of such printers is relatively straightforward. In this regard, drops of a colored ink are emitted onto a print medium such as paper or transparency film during a printing operation, in response to commands electronically transmitted to a printhead. These drops of ink combine on the print medium to form the pattern of spots that make up the text and images perceived by the human eye. Inkjet printers may use a number of different ink colors. One or more printheads are mounted in a print cartridge, which may either contain the supply of ink for each printhead or be connected to an ink supply located off-cartridge for the printhead. An inkjet printer frequently can accommodate two to four such print cartridges. The cartridges are typically mounted side-by-side in a carriage which scans the cartridges back and forth within the printer in a forward and a rearward direction above the medium during printing such that the cartridges move sequentially over given locations, called pixels, arranged in a row-and-column format on the medium.
A thermal inkjet printhead typically has a substrate (preferably made of silicon or other comparable materials) with multiple thin-film heating resistors on it. Structural barriers separate the thin film resistors from each other and form a chamber into which ink flows and is heated upon selective activation of the resistors. Thermal excitation causes expulsion of the ink from the printhead through a nozzle associated with each chamber and formed on an outer nozzle member of the printhead. Initially, these nozzle members were plates manufactured from one or more metallic compositions such as gold-plated or palladium-plated nickel and similar materials. However, more recently they have been produced from organic polymers (e.g. plastics). A representative polymeric (e.g. polyimide-based) composition suitable for this purpose is a commercial product sold under the trademark “KAPTON” by E.I. du Pont de Nemours & Company of Wilmington, Del. (USA).
The set of nozzles are arranged on the printhead such that a certain width of the medium corresponding to the layout of the nozzles can be printed during each scan, forming a printed swath. The printer also has a medium advance mechanism which moves the medium relative to the printheads in a direction generally perpendicular to the movement of the carriage so that, by combining scans of the print cartridges back and forth across the medium with the advance of the medium relative to the printheads, ink can be deposited on the entire printable area of the medium. The basics of this technology are further disclosed in various articles in several editions of the Hewlett-Packard Journal [Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No. 1 (February 1994)], incorporated herein by reference.
The quality of the printed output produced by the printer is a very important feature to inkjet printer purchasers, and therefore printer manufacturers pay a great deal of attention to providing a high level of print quality. In order to provide high print quality, each nozzle of the printhead should be able to repeatably deposit the desired amount of ink in precisely the proper pixel location on the medium, producing round spots or dots. However, printhead aberrations and the effects of aging can adversely affect precise ink drop placement. The actual location of misplaced drops can visibly differ from the desired location, much like missing the bulls-eye of a target. The location error can have a component in the direction in which the print cartridge is scanned; such error is known as scan axis directionality (“SAD”) error. The location error can also have a component in the direction in which the print medium is advanced; such error is often called paper axis directionality (“PAD”) error.
Another form of drop placement error also occurs because ink is typically not ejected from a nozzle in the form of a single drop, but rather as a main drop followed by one or more satellite drops. All of these drops would ideally be deposited in the same pixel location; however, because the main and satellite drops are ejected at slightly different times, satellite drops typically land downstream in the scan direction from the main drop. Instead of printing a round spot on the medium, non-coincident main and satellite drops can produce a non-round spot with a “tail”, or even more than one spot on the medium. As the scanning speed of the printhead with respect to the medium increases, the time separation between the main and satellite drops has a greater effect, and it becomes more likely that the main and satellite drops will not result in round spots as desired.
Drop placement errors generally cause a visually significant print quality defect known as banding: strip-shaped nonuniformities that are visible throughout the printed image. Banding is particularly noticeable when the drop placement errors are not consistent from nozzle to nozzle on the printhead. Banding is also particularly noticeable when the drop placement errors for a single nozzle vary between consecutive drops, such as when the main and satellite drops sometimes coincide, but other times don't coincide. Furthermore, a combination of round and non-round spot shapes in an area on the medium which is intended to be printed with a uniform color and intensity can result in an undesireable variation of lightness and darkness within the supposedly uniform area. Accordingly, it would be highly desirable to have a new and improved inkjet printer and method for depositing drops of ink that can be utilized to repeatably produce accurately placed round spots on the print medium at all scanning speeds.